Efimosfermin Alfa
Low EvidenceEfimosfermin alfa (development code BOS-580) is an investigational, long-acting, engineered analog of the metabolic hormone fibroblast growth factor 21 (FGF21) being developed by GSK - which acquired the molecule from Boston Pharmaceuticals in May 2025 for $1.2 billion upfront and up to $800 million in milestones (potential ~$2 billion total) - for steatotic liver disease, principally metabolic dysfunction-associated steatohepatitis (MASH, formerly NASH). Its defining feature is a very long half-life engineered to allow once-monthly subcutaneous dosing - the least frequent schedule among the leading FGF21 analogs, contrasting with the once-weekly or every-two-weeks dosing of efruxifermin (an Fc-FGF21 fusion) and pegozafermin (a glycoPEGylated FGF21). Like the rest of the class, it activates FGF receptors (FGFR1c/2c/3c) together with the co-receptor beta-Klotho to cut liver fat, improve insulin sensitivity, lower triglycerides, and drive direct anti-inflammatory and anti-fibrotic effects in the liver. In a 24-week Phase 2 trial in biopsy-confirmed F2-F3 MASH, once-monthly efimosfermin achieved at least one-stage fibrosis improvement without worsening of MASH in about 45% of patients and MASH resolution without worsening of fibrosis in about 68%, both significantly better than placebo (published in The Lancet, 2025). It holds FDA Breakthrough Therapy designation and EMA PRIME (Priority Medicines) status for MASH, and has advanced into the Phase 3 ZENITH program (ZENITH-1 and ZENITH-2 in F2-F3 MASH), with a Phase 3 program in compensated (F4) MASH cirrhosis also underway; GSK has guided to a first potential launch around 2029.
What It Is
Efimosfermin alfa (BOS-580) is an investigational, long-acting analog of fibroblast growth factor 21 (FGF21), the endogenous hormone the body releases - mainly from the liver - under fasting and metabolic stress to tell liver, fat, and brain tissue to burn fat, sharpen insulin sensitivity, lower circulating triglycerides, and quiet hepatic inflammation and scarring. Native FGF21 is an appealing target for metabolic dysfunction-associated steatohepatitis (MASH), the fibrosing form of fatty liver disease that can progress to cirrhosis, but it is cleared from the body within hours, making it impractical as a drug. Efimosfermin is engineered to dramatically extend that half-life - far enough to support once-monthly subcutaneous injection, the longest dosing interval among the front-running FGF21 analogs and its main practical differentiator from once-weekly efruxifermin and once-weekly/every-two-weeks pegozafermin. Mechanistically all three converge on the same pathway: the FGF21 backbone anchors to the co-receptor beta-Klotho and then engages FGF receptors (FGFR1c, FGFR2c, FGFR3c) to trigger the metabolic signaling that reduces liver fat and fibrosis. The molecule originated at Boston Pharmaceuticals and was acquired by GSK in May 2025 in a deal worth up to roughly $2 billion, giving GSK a Phase 3-ready, potential best-in-class entrant in the increasingly crowded MASH field. Its clinical program spans pre-cirrhotic F2-F3 disease (the Phase 3 ZENITH-1 and ZENITH-2 trials) and compensated F4 cirrhosis, positioning efimosfermin as a liver-directed therapy distinct from - and a potential future combination partner for - the incretin (GLP-1/GIP/glucagon) weight-loss drugs.
Regulatory Status
Why Researchers Study It
Efimosfermin is one of the three most advanced FGF21 analogs and the one built around the longest dosing interval - once monthly - making it a key test of whether a convenient, low-burden FGF21 therapy can match the fibrosis-reversing ambitions of once-weekly rivals efruxifermin and pegozafermin. Because it works through a mechanism entirely different from the GLP-1/GIP/glucagon incretin drugs, it is studied both as a standalone MASH therapy across the fibrosis spectrum and as a potential future combination partner for incretin-based weight-loss agents. GSK's ~$2 billion acquisition and the drug's FDA Breakthrough Therapy and EMA PRIME designations have made its Phase 3 ZENITH readouts one of the more closely watched events in the MASH field, and the head-to-head question of monthly efimosfermin versus weekly efruxifermin and pegozafermin is a defining debate for how best to drug the FGF21 pathway.
Proposed Mechanisms
- FGF21 receptor agonism: efimosfermin activates FGF receptor complexes (FGFR1c, FGFR2c, and FGFR3c) together with the obligate co-receptor beta-Klotho in liver, adipose tissue, and other metabolic tissues, reproducing and amplifying the actions of native FGF21.
- Direct hepatic effects: reduces hepatic de novo lipogenesis and liver fat and drives anti-inflammatory and anti-fibrotic actions in the liver, aiming to resolve steatohepatitis and improve fibrosis.
- Improved insulin sensitivity and lipid handling: enhances peripheral glucose uptake and fat oxidation and lowers circulating triglycerides while improving the cholesterol profile, targeting the metabolic drivers of MASH.
- Adipose-tissue signaling: acts on fat tissue to raise adiponectin and promote healthier lipid storage, part of the broader FGF21 metabolic program.
- Engineered long half-life: protein engineering greatly extends the very short half-life of native FGF21 - far enough to allow once-monthly subcutaneous dosing, the longest interval among the leading FGF21 analogs and a contrast with efruxifermin's Fc-fusion and pegozafermin's glycoPEGylation weekly approaches.
Evidence Snapshot
| Study Type | Model | Outcome | Link |
|---|---|---|---|
| RCT (human, Phase 2 - 24 weeks) | Biopsy-confirmed F2-F3 MASH; once-monthly subcutaneous efimosfermin alfa (300 mg every 4 weeks) vs placebo | At least one-stage fibrosis improvement without worsening of MASH in ~45% of patients and MASH resolution without worsening of fibrosis in ~68%, both significantly greater than placebo, alongside reductions in liver fat and liver-injury markers | Source |
| RCT (human, Phase 2a - dose-ranging, 12 weeks) | Phenotypic MASH (BMI 30-45), 12 US centers; efimosfermin 75 mg or 150 mg every 2 or 4 weeks, or 300 mg every 4 weeks, vs placebo | Favorable safety and tolerability with improvements in hepatic steatosis (liver fat) and metabolic markers; results supported feasibility of every-4-week (once-monthly) dosing and further MASH development | Source |
| RCT (human, Phase 3 - ZENITH program, ongoing) | ZENITH-1 and ZENITH-2 pivotal trials in biopsy-confirmed F2-F3 MASH (e.g., NCT07221227, NCT07221188), plus a Phase 3 program and Phase 2 study (NCT06920043) in compensated F4 MASH cirrhosis | Pivotal program designed to confirm fibrosis improvement and MASH resolution across the fibrosis spectrum; no efficacy readouts yet, with a first potential launch guided to around 2029 | Source |
Commonly Discussed Benefits
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Safety & Cautions
- Investigational and not approved: efimosfermin is in Phase 3 development for MASH and is not available by prescription; its ability to improve fibrosis and clinical outcomes in the pivotal ZENITH trials remains to be confirmed.
- No self-sourcing: efimosfermin is a proprietary, engineered FGF21 biologic studied only in controlled trials. Material sold online as 'efimosfermin', 'BOS-580', or 'FGF21' is not the clinical drug and cannot be assumed authentic, pure, or safe.
- Gastrointestinal effects and appetite: the most commonly reported adverse effects in trials are diarrhea, nausea, and increased appetite, generally mild-to-moderate and transient.
- FGF21-class considerations: agents in this class have been examined for possible effects on bone-turnover markers and other parameters; the long-term significance of these signals is still being studied.
- Not a weight-loss or obesity drug: unlike the incretins, efimosfermin is being developed for liver disease (MASH), and it should not be conflated with GLP-1/GIP-based weight-loss therapies.
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Citations
- [1] Once-monthly efimosfermin alfa (BOS-580) in MASH with F2 or F3 fibrosis: 24-week Phase 2 results - The Lancet (2025) PubMed
- [2] Efimosfermin alfa (BOS-580), a long-acting FGF21 analogue, in phenotypic MASH: Phase 2a trial - The Lancet Gastroenterology & Hepatology (2025) PubMed
- [3] GSK to acquire efimosfermin, a Phase III-ready potential best-in-class medicine for steatotic liver disease - GSK (May 2025) PubMed
- [4] GSK's investigational liver therapy efimosfermin receives US FDA Breakthrough Therapy and EMA PRIME designations for MASH - GSK PubMed
- [5] A Pivotal Clinical Study to Investigate Efimosfermin Alfa in Participants With Biopsy-confirmed F2- or F3-stage MASH - ClinicalTrials.gov (NCT07221227) PubMed
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Pegozafermin
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Efruxifermin
Low EvidenceEfruxifermin (development codes EFX, AKR-001; originally AMG 876) is an investigational, once-weekly, subcutaneously injected analog of the metabolic hormone fibroblast growth factor 21 (FGF21) being developed by Akero Therapeutics for metabolic dysfunction-associated steatohepatitis (MASH, formerly NASH), including both pre-cirrhotic fibrosis and compensated (F4) cirrhosis. It is a bivalent Fc-FGF21 fusion protein - two engineered FGF21 sequences fused to a human immunoglobulin (IgG1) Fc domain - a design distinct from the glycoPEGylation used by its main FGF21-class rival, pegozafermin. The Fc fusion extends the very short half-life of native FGF21 to support once-weekly dosing while preserving agonism at FGF receptors (FGFR1c/2c/3c) together with the co-receptor beta-Klotho. In the 96-week Phase 2b HARMONY trial in biopsy-confirmed F2-F3 MASH, at least a one-stage improvement in fibrosis without worsening of MASH was reached by about 75% of patients on 50 mg and 46% on 28 mg versus 24% on placebo (published in The Lancet, August 2025). In the Phase 2b SYMMETRY trial in compensated MASH cirrhosis, about 39% of 50 mg patients with paired week-96 biopsies achieved reversal of cirrhosis without worsening of MASH versus 15% on placebo (published in the New England Journal of Medicine). Efruxifermin holds FDA Breakthrough Therapy designation and has advanced into the Phase 3 SYNCHRONY program (SYNCHRONY Histology in F2-F3 MASH, SYNCHRONY Outcomes in F4 compensated cirrhosis, and SYNCHRONY Real-World in non-invasively diagnosed MASH/MASLD, whose double-blind portion completed enrollment with safety results expected in 2026).